API to add/remove items from collections

[srosset81]

Use a PATCH method similar to ldp.resource.patch, but only allow the modification of items. Using application/sparql-update for the Content-Type.

Example request:

PREFIX as: <https://www.w3.org/ns/activitystreams#>
INSERT DATA {
  <https://mypod.store/alice/followers> as:items <https://mypod.store/bob> .
};
DELETE DATA {
  <https://mypod.store/alice/followers> as:items <https://mypod.store/craig> .
}